MECHANICS. 



29 



Buchanan that the same quantity of 

 human labour employed in working a 

 pump, turning a \vinch, ringing a bell, 

 and rowing a boat, are as the numbers 

 100, 167, -2-27, and 248. 



The most advantageous manner of 

 applying human strength is in the act 

 of rowing. 



The most useful of quadrupeds, as a 

 mechanical agent, is the horse. The 

 relative values of the labour of a horse 

 and man are variously stated. Some 

 estimate them as five to one, some six 

 to one, and some seven to one. Per- 

 haps the medium may be nearest to 

 the true average, and "that we may ge- 

 nerally consider six men equivalent to 

 one horse. 



The most advantageous method of 

 using the strength of the horse is in the 

 act of drawing. The worst method in 

 which this animal can be employed is 

 in carrying a weight up a steep hill ; 

 while, on the other hand, the peculiar dis- 

 position of the limbs of a man, renders 

 him well-fitted for this species of la- 

 hour. It has been observed that three 

 men climbing a hill, loaded with 1 OOlbs. 

 each, will ascend with greater speed 

 than one horse carrying 3 OOlbs. 



CHAPTER VIII. O?i the Mechanical 

 Agents depending on Heat. 



(58.) IN order to explain the several ways 

 in which heat is rendered subservient to 

 the production of mechanical agency, it 

 will be necessary, in the first instance, 

 to offer a few observations on its pro- 

 perties, and particularly those properties 

 which have relation to that quality of 

 -matter called cohesion. The necessity 

 of entering into very minute details on 

 this subject, however, is superseded by 

 our treatise on heat, to which we refer 

 the reader who desires to proceed with 

 the subject beyond the general view of 

 it which we shall give. 



There is supposed to exist between 

 the particles of matter, whatever be 

 their form or situation, a certain mu- 

 tual attraction, by which, if it be un- 

 resisted by any opposing force, they 

 have a tendency to approach each other, 

 io collect together, and to form them- 

 selves into solid concrete masses. Heat, 

 or caloric, is supposed to be a subtle 

 and highly elastic fluid, which trans- 

 fuses itself through the dimensions of 

 bodies in a greater or less degree, and 

 by its intense elasticity has a tendency 



to'force the particles asunder. What- 

 ever be the nature of heat, however, and 

 whether it be material or not, it is an 

 undisputed fact, that it is a cause, which 

 produces an effect exactly opposite to 

 the effects of cohesion, and that, in pro- 

 portion as it pervades any body, it gives 

 the particles of that body a tendency to 

 repel each other and fly asunder, which 

 tendency, in some cases, prevails over 

 the cohesive force and actually produces 

 that effect. 



When we find a body in the solid 

 state, we therefore conclude, that the 

 cohesive force by which its particles at- 

 tract each other greatly predominates 

 over the repulsive energy of the caloric 

 which may pervade its dimensions, and 

 that, consequently, the particles cohere 

 with a force equal to the difference 

 between these cohesive and repulsive 

 forces. If, then, by the external appli- 

 cation of fire, we transfuse through 

 the dimensions of the body an increased 

 quantity of heat, we naturally expect 

 that, the repulsive effect of the caloric 

 being increased, the particles which 

 compose the body will be more sepa- 

 rated, and will retire from each other 

 to increased distances, so as to enlarge 

 the dimensions of the body. 



This effect we find actually to obtain ; 

 for if a cylindrical bar of metal, C D* 

 be gauged by means of a flat piece 

 of metal b, (fig. 27.) furnished with a 



circular hole, in which the bar exactly 

 passes, and having a notch in its side, 

 corresponding to the length of the bar, 

 it will be found that, after the bar is 

 heated, its length and thickness will 

 be so much increased that it will no 

 longer fit in the notch, nor pass through 

 the circular aperture. 



In general, when heat is communi- 

 cated to solids, their bulk is increased 

 from the cause which we have assigned ; 

 but this effect is more perceivable in 

 metals than other solids. 



This effect of heat, however, is not 

 confined to solids, but is observable in 

 liquids, and still more in aeriform sub- 

 stances. The thermometer is an in- 

 strument in which the expansion of a 



